1. Field of the Invention
The present invention relates to a PCM recording and reproducing apparatus wherein an analog audio signal is converted to digital data for recording it on a record medium and digital data reproduced from the record medium is reconverted to the original analog audio signal, and more particularly to a muting circuit for muting noise output produced by dropouts due to defects of the record medium or failure or loss of data synchronizing signal.
2. Description of the Prior Art
Defects of record medium such as magnetic tape are unavoidable due to deposition of fine dusts on the medium or damage of the medium during its manufacture or usage, and phenomenon called dropout is encountered. If even a bit of digital data is erroneously coded or decoded due to the dropout, the data is varied to an unexpected value and the reproduced sound includes a large clicking noise.
A known method for overcoming the noise problem is to provide, in addition to the digital data representative of the analog audio signal, an error correction code for correcting error when errors are included in the digital data and a cyclic redundancy check code (hereinafter referred to as CRC) for detecting the errors so that the errors in the digital data are detected by the CRC and the correct data is restored by the error correction dode to prevent the noise.
This method is effective to a small dropout. In the PCM recording the reproducing apparatus, however, a considerably long dropout may occur resulting in the error or loss of a number of data bits. In such a case, the error correction code cannot correct the errors in the data and consequently noise may be produced.
On the other hand, as to synchronizing signals, horizontal/vertical synchronizing signals and synchronizing signal in the PCM recording the reproducing apparatus are locked in a PLL (phase locked loop) circuit. As a result, the locking is unlocked when a power supply is turned on or by the dropout, and the synchronizing signals are lost to cause discontinuity of data. This discontinuity of data and the resulting erroneous correction of data may also produce noise. This is remarkable when data synchronizing signal is lost continuously.
When a VTR tape is used as a record medium in the PCM recording and reproducing apparatus and a VTR magnetic tape having picture image signal recorded thereon is erroneously loaded to a VTR, big noise will be produced because the picture image data is treated as the PCM audio data. When such a big noise is inputted, it is difficult to reproduce correct data because it is beyond the correction ability of the error correction code and the CRC.